The present invention relates generally to tools used in subterranean wells and, in a preferred embodiment thereof, more particularly provides a proppant slurry screen apparatus for use in formation fracturing operations.
Oftentimes, a potentially productive geological formation beneath the earth's surface contains a sufficient volume of valuable fluids, such as hydrocarbons, but also has a very low permeability. "Permeability" is a term used to describe that quality of a geological formation which enables fluids to move about in the formation. All potentially productive formations have pores, a quality described using the term "porosity", within which the valuable fluids are contained. If, however, the pores are not interconnected, the fluids cannot move about and, thus, cannot be brought to the earth's surface.
When such a formation having very low permeability, but a sufficient quantity of valuable fluids in its pores, is desired to be produced, it becomes necessary to artificially increase the formation's permeability. This is typically accomplished by "fracturing" the formation, a practice which is well known in the art and for which purpose many methods have been conceived. Basically, fracturing is achieved by applying sufficient pressure to the formation to cause the formation to crack or fracture, hence the name. The desired result being that the cracks interconnect the formation's pores and allow the valuable fluids to be brought out of the formation and to the surface.
A conventional method of fracturing a formation begins with drilling a subterranean well into the formation and cementing a protective tubular casing within the well. The casing is then perforated to provide fluid communication between the formation and the interior of the casing which extends to the surface. A packer is set in the casing to isolate the formation from the rest of the wellbore, and hydraulic pressure is applied to the formation via tubing which extends from the packer to pumps on the surface.
The pumps apply the hydraulic pressure by pumping fracturing fluid down the tubing, through the packer, into the wellbore below the packer, through the perforations, and finally, into the formation. The pressure is increased until the desired quality and quantity of cracks is achieved. Much research has gone into discerning the precise amount and rate of fracturing fluid and hydraulic pressure to apply to the formation to achieve the desired quality and quantity of cracks.
The fracturing fluid's composition is far from a simple matter itself. Modern fracturing fluids may include sophisticated man-made proppants suspended in gels. "Proppant" is the term used to describe material in the fracturing fluid which enters the formation cracks once formed and while the hydraulic pressure is still being applied (that is, while the cracks are still being held open by the hydraulic pressure), and acts to prop the cracks open. When the hydraulic pressure is removed, the proppant keeps the cracks from closing completely. The proppant thus helps to maintain the artificial permeability of the formation after the fracturing job is over. Fracturing fluid containing suspended proppant is also called a slurry.
A proppant may be nothing more than a very fine sand, or it may be a particulate material specifically engineered for the job of holding formation cracks open. Whatever its composition, the proppant must be very hard and strong to withstand the forces trying to close the formation cracks. These qualities also make the proppant a very good abrasive. It is not uncommon for holes to be formed in the protective casing, tubing, pumps, and any other equipment through which a slurry is pumped.
Particularly susceptible to abrasion wear from pumped slurry is any piece of equipment in which the slurry must make a sudden or significant change in direction. The slurry, being governed by the laws of physics, including the principles of inertia, tends to maintain its velocity and direction of flow, and resists any change thereof. An object in the flowpath of the slurry which tends to change the velocity or direction of the slurry's flow will soon be worn away as the proppant in the slurry incessantly impinges upon the object.
Of particular concern in this regard is the piece of equipment attached to the tubing extending below the packer which takes the slurry as it is pumped down the tubing and redirects it radially outward so that it exits the tubing and enters the formation through the perforations. That piece of equipment is known to those skilled in the art as a crossover. Assuming, for purposes of convenience, that the tubing extends vertically through the wellbore, and that the formation is generally horizontal, the crossover must change the direction of the slurry by ninety degrees. Because of this significant change of direction, few pieces of equipment (with the notable exception of the pumps) must withstand as much potential abrasive wear as the crossover.
In addition, the crossover is frequently called upon to do several other tasks while the slurry is being pumped through it. For example, the crossover typically contains longitudinal circulation ports through which fracturing fluids that are not received into the formation after exiting the crossover are transmitted back to the surface through an annular area between the tubing and the wellbore casing above the packer.
Space limitations in the wellbore dictate that the circulation ports are not far removed from the flowpath of the slurry through the crossover. If the crossover is worn away such that the slurry flowpath achieves fluid communication with the circulation ports in the crossover, the fracturing job must cease while the tubing is removed from the wellbore to replace the crossover at great loss of time and money. Otherwise, the slurry may enter the circulation ports in the crossover, flow into the annular area between the tubing and the wellbore casing above the packer or a screen annulus area below the packer, and possibly stick the tool in the well.
A major reason for the service tool sticking when the crossover becomes eroded between the slurry flowpath and the circulation ports is that, unless the pumps are stopped immediately after the slurry flowpath achieves fluid communication with the circulation ports, the slurry will be pumped through the circulation ports to the annular area above the packer. This results in the proppant in the slurry settling out in the annular area above the packer and around the tubing from which the service tool is suspended. It is not uncommon for such pumping of the slurry into the annular area above the packer to go unnoticed and, consequently, large volumes of proppant to be deposited around the tubing.
For the above reasons and others, the crossover has commonly been considered a critical piece of equipment, whose failure during slurry delivery usually means failure of the entire fracturing job. Extensive measures have been employed in the past to avoid failure of the crossover, that is, to retard abrasive wear of the crossover and the resultant communication between the slurry flowpath and circulation ports. None, however, have solved the problem of how to prevent sticking of the service tool after the crossover has failed by preventing the deposition of proppant in the annular area above the packer.
From the foregoing, it can be seen that it would be quite desirable to provide a proppant slurry screen apparatus which prevents depositing of proppant around the tubing above or below the packer following the failure of the crossover. It is accordingly an object of the present invention to provide such a proppant slurry screen apparatus and associated methods of using same.